(1) Field of the Invention
The present invention relates to a blade damper, and to a rotor fitted with such a damper.
More particularly, the invention lies in the technical field of damping the drag movements of a blade, e.g. a blade of the rotary wing of a rotorcraft.
(2) Description of Related Art
A rotorcraft rotor conventionally comprises a hub driven in rotation about an axis of rotation by an outlet shaft from a power gearbox, together with at least three blades fastened to the hub via suitable hinges, in particular via a dedicated laminated spherical abutment for each blade.
The oscillations of each blade about its drag axis can become coupled in unstable manner with the resilient movements or deformation modes of the airframe, in particular the oscillations of the helicopter when standing on the ground via its landing gear: this is the origin of the so-called “ground resonance” phenomenon that can be dangerous for the aircraft when the resonant frequency of the oscillations of the blades about their drag axes is close to one of the resonant frequencies of the oscillations of the aircraft on its landing gear.
It should be observed that in the field of rotorcraft, other resonance phenomena are known under the terms “air resonance” and “drive train resonance”.
The remedies to resonance phenomena due to the movements of the blades of a rotorcraft rotor consist in damping the movements of the blades relative to their drag axes by means of a damper type device.
Such dampers comprise resilient return means of determined stiffness and damping characteristics for opposing the resonance phenomena, in particular ground resonance and drive train resonance, that appear in particular on helicopters.
When drag movements of the blades of the rotor are excited, the blades depart from their equilibrium positions and may become distributed non-uniformly in the circumferential direction, thereby creating an unbalance by moving the center of gravity of the rotor away from its axis of rotation. Furthermore, the blades that have departed from their equilibrium positions then oscillate about their positions at a frequency ωδ, which is the resonant frequency of the blades in drag, also known as the first drag mode or the resonant mode in drag.
If Ω is the frequency of rotation of the rotor, it is known that the helicopter fuselage is thus excited at the frequencies |Ω±ωδ|.
When standing on the ground via its landing gear, the fuselage of the helicopter constitutes a mass system suspended above the ground by a spring and a damper in each undercarriage. The fuselage resting on its landing gear is thus mainly characterized by its resonant modes of vibration in roll and in pitching. There is a risk of instability on the ground when the excitation frequency of the fuselage on its landing gear is close to the resonant frequency of oscillation |Ω+ωδ| or |Ω−ωδ|, which corresponds to the phenomenon known as ground resonance. To avoid instability, it is known firstly to attempt to avoid those frequencies intersecting, and if such intersection cannot be avoided, then it is necessary to damp the fuselage on its landing gear and also the blades of the main rotor in their drag movements.
Consequently, the stiffness of the drag dampers of the blades of a main rotor needs to be selected so that the resonant frequency of the blades in drag lies outside a potential zone of ground resonance, while also providing sufficient damping, since when the speed of rotation of the rotor passes through the critical speed where resonant frequencies intersect, both when speeding up and when slowing down the rotor, the movements of the blades must be damped sufficiently to avoid entering into resonance.
That is why drag dampers with resilient return means of determined stiffness are also known as frequency adapters.
In general, the stiffness of a damper associated with a blade introduces equivalent angular stiffness opposing the angular movements of the blade relative to the hub about its drag axis. It is thus possible to increase the frequency of the resonant mode of the blades in drag so as to move this frequency away from the two above-mentioned resonance phenomena.
The equivalent angular stiffness is proportional to the square of the lever arm between the damper and the drag axis of the blade, i.e. the distance between the drag axis and the axis passing through the centers of the two ball joints of the damper, where ball joints are necessary in this application.
Document FR 2 653 405 describes two different configurations for such dampers.
Thus, according to that document, the hub of a rotor comprises an annular central portion, an intermediate portion having one cavity per blade, and then a peripheral portion.
Each blade is then secured via its root to a cuff that is fastened to a laminated spherical abutment arranged in one of said cavities.
In addition, in a first embodiment, one rotary damper per blade is secured to the peripheral portion of the hub. The rotary damper is then a return member incorporating damping as a result of shear in a viscoelastic material presenting a high degree of remanence to deformation and extending between a stationary strength member and a movable strength member.
It should be observed that document FR 2 592 449 presents a rotary damper having a return member that uses a fluid.
In order to be able to damp the drag movement of a blade, the rotary damper is connected by a connecting rod to a blade cuff.
That first embodiment is suitable for rotorcraft having a rotor with three blades.
Since the lever arm of that device is relatively small, it is appropriate to implement a rotary damper that is overdimensioned and therefore bulky, which limits any possibility of use with a large number of blades.
Thus, for rotorcraft having a rotor with at least four blades, document FR 2 653 405 proposes a second embodiment.
In accordance with that second embodiment, a rotary damper is fastened inside each cuff, the rotary damper of a blade being connected to an adjacent blade by a connecting rod.
Compared with a more conventional configuration in which the dampers are interposed between the blades and the hub of the rotor, arranging the dampers between the blades serves to increase the lever arm between the dampers and the drag axes of the blades, and also causes two dampers per blade to participate in opposing ground resonance. The stiffness of each damper can therefore be limited accordingly, and one advantage that stems therefrom is a lower level of static force being introduced by mounting each of the dampers as an inter-blade adapter. Such a configuration is therefore very favorable for combating ground resonance.
However, the inter-blade configuration does not enable overall drag movements to be damped and requires drag abutments to be used in order to avoid damage on starting and above all on breaking the rotor. Furthermore, it may be necessary to take particular precautions in order to avoid entering into resonance with the drive train.
Consequently, the prior art provides two distinct and alternative embodiments, namely:                a first arrangement of one rotary damper between the hub and a cuff; or        a second arrangement of one rotary damper between two blades.        
Each of those embodiments presents its own advantages and drawbacks.
The first arrangement is efficient in combating drive train resonance in particular. Nevertheless, it uses only one damper per blade. A failure or breakage of a damper can then be penalizing.
The second arrangement in fact has two dampers per blade, thereby increasing safety. However, collective drag movement of the blades takes place without stiffness and without damping, which can give rise to a problem of drive train resonance.
Depending on requirements, it is either the first arrangement or the second arrangement that is selected.
The state of the art also includes the following documents: EP 1 767 452; WO 94/15113; FR 2 929 675; and U.S. Pat. No. 4,580,945.